1. Field of the Invention
This invention relates to rotary positive displacement pumps of the internally-meshing screw kind in which a helical rotor, e.g., a member of circular section formed into a helix or a member having an external helical shape, is rotated inside a stator with a rounded helical groove, the rotor longitudinal axis being radially offset from the stator longitudinal axis and the rotor and stator being co-operatively shaped so that the rotor has moving contact with the stator around the helical groove in the stator to form a displacement pump. Such a pump is well-known and will hereinafter be referred to as a rotary positive displacement pump of the internally-meshing screw kind.
2. Prior Art
The stator of such a pump is usually formed of an elastomer, such as a hard rubber, whilst the rotor is preferably of metal. In a pump of this kind, the rotor does not turn about its axis but has to be moved around a circular path as it rotates. It is therefore the common practice to drive such rotor from a drive shaft by means of a connecting shaft which is coupled at one end by a first universal joint to one end of the rotor and is coupled at its other end by a second universal joint to the drive shaft. Since the material to be pumped has to pass axially through the stator from one end to the other, the connecting shaft with the universal joints is commonly in the inlet or outlet chamber for the fluid being pumped. This causes problems when particulate material, particularly abrasive material has to be pumped; flexible protective covering have to be provided for the universal joints.